About two and one-half billion years ago, life on Earth was still in its infancy. Complex organisms such as plants and animals had not yet appeared, but the planet was teeming with microscopic bacteria which thrived in the temperate and nutrient-rich environment. Greenhouse methane lingered in the atmosphere and trapped the sun's warmth, creating a climate very accommodating to the stew of microbes life that made their home on primitive Earth.
But a billion years of bacterial evolutionary progress was soon stunted by a catastrophic global event. Geologists find no signs of a great meteor impact nor a volcanic eruption, but they have uncovered the unmistakable geologic scars of rapid worldwide climate change. Average temperatures, which were previously comparable to our present climate, plummeted to minus 50 degrees Celsius and brought the planet into its first major ice age. This environmental shift triggered a massive die-off which threatened to extinguish all life on Earth, and paleoclimatologists have good reason to believe that this world-changing event was unwittingly caused by some of the planet's own humble residents: bacteria.
The period in history is known as the Paleoproterozoic era, and prior to that time the Earth's ecology and environment were significantly different. The iron-rich waters of the oceans lent them a green tint, and the atmosphere was comprised of gasses other than oxygen. Although oxygen atoms were abundant, such as those found in water molecules, unbound oxygen was extremely rare. The sea was host to a plethora of anaerobic microorganisms, but there were also a few members of a newly evolved variety: a blue-green algae known as cyanobacteria. These adapted bacteria were the first to use water and sunlight for photosynthesis, producing oxygen as a by-product of their metabolism.
The cyanobacteria were a struggling minority at first, but scientists believe that these new microbes began to dominate with the help of meltwater from a few glaciers scattered across the young continents. These glaciers spent centuries scraping across the Earth collecting minerals, ultimately depositing their rich nutrient payloads into the oceans. The cyanobacteria flourished in the presence of the increased minerals, and the rapidly growing population was soon venting increasingly large amounts of its poisonous waste oxygen into the environment.
At first the damage was limited to the oceans' ecosystems. The underwater oxygen began to chemically react with the abundant iron, eventually scrubbing the seas clean of the element through oxidation. The oxidized iron settled to the ocean floor, and the oceans' green tint began to fade. This series of developments was nothing short of an ecological disaster-- oxygen was poisonous to most of primitive Earth's inhabitants, and many bacteria relied on the iron as a nutrient.
Once the oceans' supply of iron was exhausted, oxygen began to seep from the sea into the air. With very little competition for resources, cyanobacteria continued to proliferate and pollute. The free oxygen they produced reacted with the air, gradually breaking down the methane which kept the Earth's atmosphere warm and accommodating. It took at least a hundred thousand years-- a short duration in geological terms-- but the Earth was eventually stripped of her methane, and with it her ability to store the heat from the sun. Temperatures fell well below freezing worldwide, and a thick layer of ice began to encase the oxygen-saturated planet.
Not even cyanobacteria were immune to the effects of this major ice age. The traits which had once given them such an evolutionary advantage were creating an environment which was completely inhospitable, even for themselves. As the centuries marched on, the surface became increasingly cold and frozen, with the ice at the equator eventually reaching up to one mile in thickness: Earth was an ice planet. Thermal vents on the ocean floor provided pockets where some resilient bacteria managed to survive, and certain organisms which lived underground were insulated from much of the destruction; but these reservoirs of life were scarce. Almost every living thing on Earth died as a result of this massive bacteria-induced climate change, an event known as the oxygen catastrophe.
As told by the Earth's ancient rocks, the story of the Paleoproterozoic era is one of near-extinction for all life on the planet. The rocks that lined the ocean floor during that period are layered with oxidized iron... the remains of the iron that was removed by the oxygen. Layers from previous periods have no such banded iron formations. The fossilized microbes in the rocks are also indicative of violent climate change.
The survivors of the oxygen catastrophe eventually adapted to consume the abundant oxygen and produce carbon dioxide. This greenhouse gas very gradually made its way into the atmosphere, increasing in concentration and nudging temperatures back into the hospitable range over millions of years. Had temperatures been slightly colder during the first major ice age-- if Earth had been in a slightly more distant orbit-- the planet would have remained an icy wasteland because the carbon dioxide would have frozen solid before it could promote the greenhouse effect.
Temperatures reached as low as minus 50 degrees Celsius, and carbon dioxide freezes into dry ice at minus 78 degrees. Indeed it seems that life on Earth was spared by a very tiny margin.
Today all life on the planet can trace its lineage back to those few microorganisms which survived the great dying of 2,500,000,000 BC, and now cyanobacteria are among the most common bacteria on Earth. In the billions of years since the first ice age, the environment has been dramatically altered on numerous occasions by greenhouse gases which trap heat; by shifting tectonic plates which reroute ocean currents; by our sun's varying radiation levels; and by volcanic activity which alters the atmosphere. But at least once in Earth's long history, its own occupants seem to have unwittingly brought all life to the brink of extinction. The sun is warmer now than it was then, so such a "Snowball Earth" is a bit less likely to occur... but the cautionary tale catalogued in ancient rocks warns us that the environment is certainly not impervious to the actions of those living in it.